Alloys of lead and copper have been used for many years as bearings, such as in the automotive field. The alloy is considered to have a soft phase, which is substantially lead, reinforced by a network of copper. Apparently, in a lead-copper bearing, the lead squeezes out to form a film of soft metal on the surface of the hardener copper, and thus gives valuable frictional properties with a coefficient of friction lower than that of either the lead or copper. The alloy is, of course, quite dense and is often used as a lining on steel sheets. Some of the very facts that make the lead-copper alloys a good bearing material, make it a very difficult material to form or cast. The metals have limited liquid miscibility; there is an absence of the formation of compounds; and there is very limited solid solution. Segregation of the components of the alloy is a particular problem.
In contrast to the lead-copper alloy, aluminum is used largely as a structural metal due to its light weight, strength and excellent corrosion resistance. Attempts to alloy aluminum with other metals such as lead or copper have met with difficulties due to the insolubility of these metals in aluminum. The solubility of copper in aluminum is only 0.1% at room temperatures, but it is known that a somewhat larger amount can be kept dissolved in aluminum by addition in a molten state followed by rapid cooling. However, reheating of the alloy will then cause precipitation of the copper.
The difficulties encountered with lead are even greater since lead in the liquid state is not miscible with aluminum if the lead content exceeds 1.8% and the solubility of lead in aluminum is very low. Moreover, solid lead forms spheres within the body of solidified aluminum which break the continuity of the metal, particularly during machining.
It is, therefore, an object of this invention to provide an improved alloy comprising a major amount of aluminum and minor but significant amounts of lead and copper.
It is an object of the present invention to provide for an improved alloy which will overcome several problems which heretofore have existed in this art.
A further object is to provide improved processes for making an alloy of aluminum, lead and copper.
This invention is also directed to aluminum-lead alloys and to methods for making such alloys.
Other objects and advantages will be evident from, and will be set forth in conjunction with, the following detailed description of the invention.